Download tarpaulin instead of building it on Travis

It's a lot faster to just get a binary that's prebuilt and works on
Travis. Previously, tarpaulin binaries would link to a
release-specific Rust library, so running it with a different
version of Rust installed would fail, but that's no longer the case.

.travis.yml

@@ -1,23 +1,12 @@
-sudo: required
 language: rust
 
-# Dependencies of tarpaulin
-addons:
-  apt:
-    packages:
-      - libssl-dev
-
-
 cache: cargo
 
 install:
   - rustup component add rustfmt-preview
   - rustfmt --version
-  - |
-    if [[ $(cargo tarpaulin --version) != *0.8.4 ]]
-    then
-      cargo install --force cargo-tarpaulin
-    fi
+  - curl -fsSL https://github.com/xd009642/tarpaulin/releases/download/0.8.6/cargo-tarpaulin-0.8.6-travis.tar.gz | tar xz
+  - mv cargo-tarpaulin ~/.cargo/bin/
 
 before_script:
   - wget https://github.com/Ortham/testing-plugins/archive/1.4.0.tar.gz